The bettas would likely eat the fry anyway, but I would stay away from mollies and platies--they prefer harder water and they're semi-aggressive, so they might not be a good match for bettas.. If I were you, I'd choose tetras or danios and cories, and skip the mollies, platies, and otos. I think the color of the females would clash with the platies also--it doesn't suit my personal aesthetic, anyway. The danios and tetras should act more like dither fish, as well. I have a school of celestial pearl danios in my big tank. They're tiny and gorgeous--they make the tank look gigantic.